Conventionally, as a method of producing a friction material for use in e.g. a disc brake pad or shoe for a vehicle, there are know a dry method and a wet method.
According to the dry method, all of raw material powders of a thermosetting resin such as a phenol resin, a fibrous base material such as a glass fibers, aramid fibers and a friction modifier such as rubber dust, cashew dust, metal powder etc. are mixed uniformly in a mixer. The resultant mixture is subject to a preforming step within a mold. Then, the formed product is transferred into a mold in which a back plate is set. The preformed product is heated and pressurized in the mold to be integrated with the back plate.
On the other hand, according to the wet method, all of the raw material powders are mixed and wetted uniformly in the presence of a solvent. The resultant wet raw material mixture is dried and then heated and pressurized for forming (e.g. Patent Document 1).
In general, raw materials for a friction material are mostly in the form of powder. Hence, e.g. when the dry method is implemented, when the mixture is removed after mixing of the friction-material raw materials, there may occur generation of dust which deteriorates the environment. Further, when powders having significantly different particle sizes are mixed altogether at one time, there may occur a segregation in which powder of large particles is deposited in the lower layer while powder of smaller particles is deposed in the upper layer, thus leading to insufficient dispersion, which makes it difficult to obtain a homogeneous friction material.
In order to prevent occurrence of such dust generation or segregation, it may be effective to implement the wet method in which mixing/stirring process is effected with charging a liquid of an organic solvent or the like together with the raw materials at the time of raw materials mixing. However, in case an organic solvent such as toluene, methanol, acetone, methyl ethyl ketone, etc. is charged, this may cause such a problem as a danger of catching a fire or an adverse effect to the human body. So, this is unfavorable in terms of working environment.
Patent Document 2, for instance, discloses a method of producing friction material in which as a liquid used in the wet method, there is employed water with no resin or the like dissolved therein, instead of the organic solvent and a phenol resin as a binder resin is formed into microcapsules.
In case water is employed in the wet method, there will occur elution of slaked lime (calcium hydroxide: Ca(OH)2) added as a pH control agent and if a phenol resin is employed as a binder resin for the friction material, there may occur a “reddening degradation” of the mixture powder obtained by mixing of friction-material raw materials, which phenomenon adversely affects the strength of the friction material. However, according to the technique disclosed in Patent Document 2, by micro-encapsulating the phenol resin as a binder resin, it is made possible to prevent this binder resin from coming into contact with water or slaked lime. Thus, the reddening degradation of the phenol resin can be prevented, whereby deterioration in the strength of the friction material can be avoided.